Brightness enhancement films (BEFs), otherwise known as prism sheets, are made by forming a prism pattern on optical substrate film. The BEFs serve to concentrate light toward the output side of a backlight, when they are incorporated onto the front surface of that backlight. The prism sheet is, in essence, a film for boosting brightness levels while keeping the power consumed a constant. FIG. 1 illustrates a common architecture employed in liquid crystal displays 1 (LCDs). A cold fluorescent light 5 and a light guideplate 6 act as a backlight to transmit light through first and second BEFs 4a and 4b, otherwise known as prism sheets, that are crossed 90° relative to each other. Prior to entering the first BEF 4a the light passes through a first diffuser 3a. Light emanating from the second BEF 4b passes through a second diffuser 3b and finally through an LCD panel 2. In the configuration illustrated in FIG. 1, the BEFs 4a,b provide an output radiation (light) pattern that is restricted to about ±21°.
The BEFs 4a,b utilize a prismatic structure to provide brightness gain. The BEFs 4a,b direct the light through the LCD panel 2, thereby providing increased brightness toward the on-axis viewer. A single sheet (e.g., the first BEF 4a) provides up to 60% increase in brightness and two sheets crossed at 90° (e.g., the first and second BEFs 4a,b as shown in FIG. 1) can provide up to 120% brightness increase. The increased brightness provides power savings. Single sheets of BEFs 4a may be used with LCD panels 2 in monitors and televisions. Crossed sheets of BEFs 4a,b may be used with LCD panels 2 in notebook personal computers.
As shown in FIG. 1, a backlight is a form of illumination used in LCDs. Backlights illuminate the LCD from the side or back. Backlights increase visibility of small and large displays in low light or bright sunlight conditions. In computer displays and LCD televisions, backlights produce light in a manner similar to a CRT display. Backlights can be color or monochrome. Color LCD displays such as those used for television or computer monitors generally use white backlights to cover most of the color spectrum.
Large area backlight systems are used in a variety of large display systems including laptop or notebook computer systems, large screen LCD screens, sunlight readable avionic/automotive displays, air traffic control displays, and medical display systems, to mention a few. Systems such as commercial aircraft cockpit displays and automotive displays including global positioning systems (GPS) navigation systems require extremely bright backlit LCD displays.
Twisted nematic (TN) and super-twisted (STN) LCD are types of displays that suffer from poor performance when viewed at wide viewing angles due to the optical characteristics of TN and STN liquid crystal materials. Color shift and decreased contrast are due to differences in the optical path length of light rays transmitted through the liquid crystal material viewed at high viewing angles versus light rays viewed at near-normal angles. LCD designers have tried to overcome this problem by careful choice of liquid crystal materials and by utilizing various internal LCD modifications.
Conventional large area backlight systems utilize an array of point sources of radiant electromagnetic energy such as light emitting diodes (LEDs) providing direct light emission to an output aperture surface of the backlight system. An optical cavity is formed between the point sources and the output aperture surface. Conventional backlight systems require relatively deep optical cavities in order to mix the light within the optical cavity. Diffusers provide better surface uniformity, but reduce the light output and decrease the overall efficiency of the backlight system. In addition, it is difficult to reduce or tailor the field of view with conventional LED/waveguide technology for backlight systems. There is a need for a new and improved backlight system to collimate the light output from the point sources and direct the collimated light to an output aperture and diffuser to spread the light output and tailor it to the output field of view.